Let us try to find the period. Compute a22: a21=3×95−20=285−20=265≡65(mod100) a22=3×65−95=195−95=100≡0(mod100) a23=3×0−65=−65≡35(mod100) a24=3×35−0=105−0=105≡5(mod100) a25=3×5−35=15−35=−20≡80(mod100) a26=3×80−5=240−5=235≡35(mod100) a27=3×35−80=105−80=25(mod100) a28=3×25−35=75−35=40(mod100) a29=3×40−25=120−25=95(mod100) a30=3×95−40=285−40=245≡45(mod100)
Alternatively, let's try to find the period for an modulo 100.
Alternatively, note that x+x1=3. The minimal polynomial is x2−3x+1=0. The roots are x=23±5. Let α=23+5, β=23−5.
Then an=αn+βn.
We are to compute a22013mod100.
Note that αβ=1.
Also, an satisfies the recurrence an=3an−1−an−2.
Let us try to find the period of an modulo 100.
Alternatively, note that an modulo 100 is periodic, and the period divides 10000 (since the recurrence is order 2, modulo m, the period divides m2−1).
Alternatively, let's compute an modulo 4 and 25 separately, then use the Chinese Remainder Theorem.
First, modulo 4: a0=2 a1=3 a2=3×3−2=9−2=7≡3(mod4) a3=3×3−3=9−3=6≡2(mod4) a4=3×2−3=6−3=3(mod4) a5=3×3−2=9−2=7≡3(mod4) a6=3×3−3=9−3=6≡2(mod4) So the sequence modulo 4 is: 2,3,3,2,3,3,2,3,3,2,… It repeats every 3 terms after the first.
Let's check: a0=2 a1=3 a2=3 a3=2 a4=3 a5=3 a6=2 a7=3 a8=3 a9=2 So the period is 3, starting from a1.
So for n≥1, an modulo 4 cycles as 3,3,2.
Now, 22013 modulo 3: 21≡2(mod3) 22≡1(mod3) 23≡2(mod3) 24≡1(mod3) So 2k cycles 2,1,2,1,… 2013 is odd, so 22013≡2(mod3).
So a22013 modulo 4: If n≡1(mod3), an≡3 If n≡2(mod3), an≡3 If n≡0(mod3), an≡2
22013≡2(mod3), so a22013≡3(mod4).
Now, modulo 25: a0=2 a1=3 a2=3×3−2=9−2=7 a3=3×7−3=21−3=18 a4=3×18−7=54−7=47≡22(mod25) a5=3×22−18=66−18=48≡23(mod25) a6=3×23−22=69−22=47≡22(mod25) a7=3×22−23=66−23=43≡18(mod25) a8=3×18−22=54−22=32≡7(mod25) a9=3×7−18=21−18=3(mod25) a10=3×3−7=9−7=2(mod25) a11=3×2−3=6−3=3(mod25) a12=3×3−2=9−2=7(mod25) So the sequence is: 2,3,7,18,22,23,22,18,7,3,2,3,7,…
It appears to repeat every 10 terms: a10=2, a11=3, a12=7, which matches a0,a1,a2. So period is 10.
Now, 22013 modulo 10: 21=2 22=4 23=8 24=16≡6 25=32≡2 26=64≡4 27=128≡8 28=256≡6 So the cycle is 2,4,8,6 (period 4).
2013÷4=503 remainder 1, so 22013≡2 (since 24k+1≡2).
So n=2 modulo 10. So a22013≡a2≡7(mod25).
Now, combine a22013≡3(mod4) and a22013≡7(mod25).
Let x≡3(mod4), x≡7(mod25).
Let x=25k+7. 25k+7≡3(mod4) 25≡1(mod4), so k+7≡3(mod4) k≡3−7≡−4≡0(mod4) So k=4m, x=25×4m+7=100m+7 So the last two digits are 07.
Therefore, the answer is 07.
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